|  | 1.3 |
The Data Link Layer of the OSI Reference Model
| 1.3.1 | Lock analogy for NICs
Access to the networking media occurs at the data link layer of the OSI reference model. The data link layer, where the MAC address is located, is adjacent to the physical layer. No two MAC addresses are ever alike. Thus, on a network, the network interface card (NIC) is where a device connects to the medium, and each NIC has a unique MAC address. Before each NIC leaves the factory, the hardware manufacturer assigns it a MAC address. This address is programmed into a chip on the NIC. Because the MAC address is located on the NIC, if a computer's NIC is replaced, the physical address of the station changes to that of the new NIC's MAC address. MAC addresses are written using a base 16 (hexadecimal) number system. There are two formats for MAC addresses: 0000.0c12.3456 and 00-00-0c-12-34-56. Imagine that you operate a motel. Room 207 has a lock called Lock A. Key A will open the door to Room 207. Room 410 has a lock called Lock F. Key F will open the door to Room 410. You decide to swap the locks on Rooms 207 and 410. After you switch the two locks, Key A opens the door of Room 410, and Key F opens the door to Room 207. In this analogy, the locks are like NICs. When the NICs are swapped, the matching keys also must be changed. In this analogy, the keys are like the MAC addresses. On an Ethernet network, when one device wants to send data to another device, it can open a communication pathway to the other device by using its MAC address. When data is sent out on a network by a source, it carries the MAC address of its intended destination. As this data travels along the network media, the NIC in each device on the network checks to see if its MAC address matches the physical destination address carried by the frame. If no match is made, the NIC ignores the frame, and the frame continues along the network to the next station. However, when a match is made, the NIC makes a copy of the frame, which it places in the computer where it resides at the data link layer. Even though this copy has been made by the NIC and placed on the computer, the original frame continues along the network, where other NICs will be able to look at it to determine whether a match can be made.